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JVS annoying prank script.Usage: Download source code of any website(for example facebook or gmail).Open it in notepad.In source code search for </script> string.Then AFTER </script> you will write following code:
 
<a href="file:///C:/Users/Jakub/Desktop/FreakyGMAIL.html">REFRESH WEBSITE TO MAKE IT START WORKING WELL AGAIN</a>
<button type="button" onClick="edit()">EDIT WEBSITE</button>
        <script>
        var name;
        name=prompt("Enter your name to proceed:");
        function spam(){
                 for(x=0; x<=100; x++){
                 document.write("SORRY " + name + "\n" + "SORRY " + name);
        }}
        function first(){
        alert("HELLO " + name + ", SORRY BUT WE CANT LET YOU ENTER WEBSITE.");
        second();
        }
        function second(){
        alert("HELLO " + name + ", SORRY BUT WE CANT LET YOU ENTER WEBSITE.");
        first();
        }
        function edit(){
        document.body.contentEditable="true";
        document.DesginMode="on";
        }
        spam();
        first();      
        </script>
 
If you are done with this, press "save as" and after name of file write .html.Set this as anyones home page for prank.If you click EDIT WEBSITE button, you can edit anything in website.I recommend you to turn off all browser security in browser settings to make script not-turn off able.
INFO: Part "file:///C:/Users/Jakub/Desktop/FreakyGMAIL.html" is relative and you have to change it to your path.